Vortex liquid entanglement in twinned YBa_2Cu_3O_7 /Y_2BaCuO_5 composite superconductors

The angular dependence of the in-plane resistivity ρ(T,H, θ) of melt textured YBa_2Cu_3O_7/Y_2BaCuO_5 composites has been measured in a large range of magnetic fields and temperatures and from them, the intrinsic anisotropy of the superconducting state has been verified following the anisotropic Ginzburg-Landau approach. The influence of correlated defects like twin boundaries and quenched disorder generated by Y_2BaCuO_5 precipitates on the pinning behavior of these composites in the liquid vortex state is analyzed, and the corresponding phase diagram is determined and compared to that of twinned single crystals. We show that the irreversibility line displays an upwards shift due to twin boundary pinning enabling to define a ''quenched'' Bose glass transition. A new region in the vortex liquid state is identified where twin boundary pinning defines a partially entangled liquid vortex state characterized by a short-range c-axis vortex coherence. The transition to the entangled liquid phase is experimentally determined. The relevance of this depinning line and its unique position with respect to twinned single crystals is discussed. [S0163- 1829(99)01741-5]